As to a chemical and physical phenomenon detecting device which is sometimes referred to as “a detecting device” hereinafter, the detecting device utilizing a floating diffusion which is sometimes referred to as “a FD section” hereinafter has been proposed as referred to patent documents 1-8.
For example, as shown in FIG. 1, such a detecting device is provided with a sensing section 10, a charge supply section 20, a charge transfer storage section 30, a charge quantity detecting section 40 and a charge eliminating section 50.
The sensing section 10 is provided with a sensing film 12 for changing a potential correspondingly to a detected object and a reference electrode 13. In accordance with the potential change of the sensing film 12, the depth of the potential well 15 is changed in a region, namely a p-type diffusion region 72 of a silicon substrate 71 faced with the sensing film 12.
The charge supply section 20 is provided with an injection diode section 21 sometimes referred to as “ID section” hereinafter and an input control gate section 23 sometimes referred to as “ICG section” hereinafter. The ID section 21 is charged with a charge. Then, with the potential of the ICG section 23 controlled, the charge in the ID section 21 is transferred to the potential well 15 of the sensing section 10.
The charge transfer storage section 30 is provided with a transfer gate section 31 sometimes referred to as “a TG section” hereinafter and a floating diffusion section 33 sometimes referred to as “an FD section”. With the voltage of the TG section 31 controlled to change a potential of a region of the silicon substrate 71 which is faced with the TG section 31, the charge charged in the potential well 15 of the sensing section 10 is transferred to the FD section 33 and stored in the FD section 33.
The charge stored in the FD section 33 is detected by a charge quantity detecting section 40. As such a charge quantity detecting section 40, a source follower type signal amplifier can be used.
The charge eliminating section 50 is provided with a reset gate section 51 sometimes referred to as “a RG section” and a reset drain section 53 sometimes referred to as “a RD section”. With the voltage of the RG section 51 controlled to change a potential of a region of the silicon substrate 71 which is faced with the RG section 51, the charge stored in the FD section 33 is transferred to the RD section 53 and discharged from the RD section 53.
The detailed structure and the behavior of such the detecting device are explained in the following by referring to a pH sensor for detecting the concentration of hydrogen ions as an example. As explained in the following, an electron is used as a charge. The subject region of the substrate 71 is doped suitably for transferring the electron.
The detecting device used for the pH sensor has an n-type silicon substrate 71. A part of the silicon substrate 71 corresponding to the sensing section 10 is made a p-type diffusion region 72. The surface of the p-type diffusion region 72 is doped to form an n-type region 73.
In the ID section 21, the FD section 33 and the RD section 53 of the silicon substrate 71, n+ regions 74, 75 and 77 are formed respectively.
On the surface of the silicon substrate 71, a protective film 81 made of silicon oxide is formed. On the protective film 81, the electrode of the ICG section 23, the electrode of the TG section 31 and the electrode of the RG section 51 are put. If each of the electrodes are applied with voltage, the potential of each region of the silicon substrate 71 faced with each of the electrodes is changed.
In the sensing section 10, the sensing film 12 made of silicon nitride is put on the protective film 81.
As referred to FIG. 2, the basic behavior of the detecting device 1 is explained in the following.
If a solution which is a detected object contacts the sensing section 10, the depth of the potential well 15 of the sensing section 10 changes in accordance with the concentration of hydrogen ions, as referred to the step (A). Namely, the larger the concentration of hydrogen ions becomes, the deeper the potential well 15 becomes. In other words, the bottom of the potential becomes high.
On the other hand, the ID section 21 is charged with a charge by lowering the potential of the ID section 21, as referred to the step (B). At the same time, the charge charged in the ID section 21 overflows the ICG section 23 to fill the potential well 15 of the sensing section 10. By the way, the potential of the TG section 31 is lower than the potential of the ICG section 23. So, the charge charged in the potential well 15 does not overleap the TG section 31 to reach the FD section 33.
Next, with the potential of the ID section 21 increased, the charge is extracted from the ID section 21. The charge slashed off by the ICG section 23 is left in the potential well 15, as referred to the step (C). Here, the charge quantity left in the potential well 15 corresponds to the depth of the potential well 15, namely the concentration of hydrogen ions which is the detected object.
Next, with the potential of the TG section 31 increased, the charge left in the potential well 15 is transferred to the FD section 33, as referred to the step (D). Thus, the charge stored in the FD section 33 is detected by the charge quantity detecting section 40, as referred to the step (E). Then, with the potential of the RG section 51 increased, the charge of the FD section 33 is evacuated to the RD section 53, as referred to the step (F). The RD section 53 is connected to VDD which absorbs the negative charge.
The circuit configuration of the pH detecting device of FIG. 1 is shown in FIG. 3. The charge in the FD section is converted into the voltage signal by a source follower circuit so that the voltage signal is outputted as the output signal from the output line OUT.